“Before the earthquake, Christchurch’s waterways were contaminated with duck and dog faeces and as a result rarely met recreational water standards,” Dr Humphrey says.

“Despite this, some people liked to collect whitebait from the Avon and Heathcote but because of ongoing repairs to wastewater infrastructure, there is still the additional risk of human faecal contamination.”

Dr Humphrey says raw sewage discharges do occur into the rivers on occasions when contractors carry out essential repairs to sewers.

A 2013 report by ESR on the Avon River water, sediment and estuarine sediment also highlights on-going water quality issues, he says.

“Water quality has improved since the active discharge period but generally does not meet the recreational water guidelines,” Dr Humphrey says.

“The silt in the river still contains E. coli and at one site the concentration found was 15 times more contaminated than during the active discharges.

“Giardia, Campylobacter and Cryptosporidium levels in the sediment have decreased since the previous study but are still present in varying levels. Giardia can cause explosive bloody diarrhoea for months.”

Giardia eggs can live for months or years in silt and can be released into the water when the silt is stirred up. It can also survive freezing, so when a block of whitebait is thawed out at home, many parts of the kitchen can be contaminated, even if the whitebait is cooked.

“People who choose to take whitebait from the Avon or Heathcote not only put themselves and others at risk but also spoil the season for other whitebaiters and consumers, as it makes us question whether any whitebait is safe to eat. I hope those people who like to whitebait can find a stand on a clean river,” Dr Humphrey says.

“All river users are also reminded that they should always avoid contact recreation for 48 hours after heavy rainfall, as this can result in sewerage and stormwater overflows into the rivers, which further reduce the quality of the waterways.”